The present invention relates to an induction heating roller device, and to a fixing device and image forming apparatus provided with the induction heating roller device.
Conventionally, heating rollers using a halogen lamp as a heat source are employed to thermally fix a toner image. However, such heat sources require a long warm up time and the heat capacity may be insufficient. Therefore, induction heating methods are being developed to resolve such problems.
As described in Japanese Laid-Open Patent Publication No. 2002-222688, the inventors of the present invention have developed an image forming apparatus and fixing device using an induction heating roller device of the transformer coupling type. The heating roller device includes a heating roller having a hollow structure in which an induction coil is air-core transformer coupled to an induction coil and rotatably supported. A secondary side resistance value of the heating roller is obtained from a closed circuit having a secondary reactance that is substantially equal to the secondary reactance. The invention conserves power used by the heating roller in induction heating and readily increases the speed of thermal fixing.
Since the induction coil is arranged in the heating roller, the temperature of the induction coil becomes high during operation. Therefore, when arranging a matching circuit or a high-frequency power source near the induction coil, the matching circuit and the high-frequency power source must have a high heat-resistance level or be protected from the heat of the heating roller and the induction coil. However, this would increase costs. Further, an increase in the heat-resistance level would enlarge the matching circuit or the high-frequency power supply. This would enlarge a fixing device or an image forming apparatus that incorporates the fixing device.
To solve this problem, the high-frequency power source and the matching circuit may be separated from the induction coil, and a high-frequency transmission line may connect the matching circuit and the induction coil.
The coupling co-efficient of the induction coil and the heating roller is normally small. This decreases the power factor of the current flowing through the induction coil and increases the power capacity (VA) of the high-frequency current flowing through the high-frequency transmission line. In addition, the wire inductance of the high-frequency transmission line between the matching circuit and the inductance coil becomes such that it cannot be ignored. This further decreases the power factor of the current flowing through the induction coil. Thus, the wire diameter of the high-frequency transmission line must be increased or the heat resistance grade of the high-frequency transmission line must be increased. This increases the cost of the high-frequency transmission line. Further, an increase in the VA of the high-frequency transmission line increases the noise emitted outward from the high-frequency transmission line. This may cause erroneous operation of the surrounding electronic circuits.
To solve this problem, the inventors of the present invention have proposed an induction coil device including an induction coil, a heating roller that is magnetically coupled to the induction coil to be heated by electromagnetic induction, a power factor improving means arranged near the induction coil, a high-frequency power source, a high-frequency transmission line, and a matching circuit. This invention decreases the VA of the high-frequency transmission line and decreases the noise emitted outward from the high-frequency transmission line thereby solving the above problem.
To increase the efficiency for transmitting power from the induction coil to the heating roller, the magnetic coupling between the induction coil and the heating roller must be strengthened. However, when the distance between the induction coil and the heating roller is decreased, the distributed capacitance between the induction coil and the heating roller becomes relatively large because distributed capacitance exists between the induction coil and the heating roller. The distributed capacitance may be several tens of pF or greater. When the frequency of the high-frequency power supplied to the induction coil is several hundred kHz or greater and the high-frequency voltage applied to the induction coil is several hundred V or greater, the leakage current resulting from the distributed capacitance increases. This exceeds the leakage current specification of an image forming apparatus in which the induction heating roller device is incorporated as a fixing device.
The leakage current produces common mode noise and causes erroneous operation of the induction heating roller device and the image forming device.